The subject invention generally concerns measurement of signals on a transmission line, and specifically concerns measurement of signals at an intermediate point of a source-terminated transmission line.
It is well known in the electronics industry to terminate a unidirectional signal transmission line (i.e., unidirectional bus) at the source end with a series impedance equivalent to the impedance of the transmission line. Series impedance source termination of unidirectional transmission lines is commonly-used by circuit design engineers because it provides a signal at the data receiver end of the transmission line that is a true representation of the signal produced at the source end of the transmission line.
Often, test engineers need to monitor signals on such buses to acquire data for use in troubleshooting the circuitry. Unfortunately, probing such a series impedance source terminated transmission line is problematic, because a signal transmitted down a transmission line by a source amplifier will reflect from the receiver end, and propagate back up the transmission line. That is, the voltage signal at any point on the line is the sum of an incident waveform component and a reflected waveform component. Thus, a signal observed at most points on the transmission line is not a true representation of the transmitted signal. As noted above, the one exception is at the far end of the line, at the input of the data receiver. Unfortunately, in many cases the input terminal of the data receiver is not accessible for probing.
What is needed is an apparatus and method which allow monitoring of a series impedance source terminated unidirectional transmission line at any point along the line and which produces at an output a signal representative of only one of the incident waveform or the reflected waveform.
Apparatus for monitoring a signal at an intermediate point on a series impedance source terminated unidirectional transmission line employs a voltage probe, a current probe, and a summing circuit. The voltage probe derives a signal from the transmission line that is representative of the sum of the incident and reflected voltage waveforms. The current probe produces a voltage signal representative of the difference between the incident and reflected waveform currents. The summing circuit algebraically adds the output signals of the voltage and current probes, and produces an output signal representative of only one of the transmitted waveform or the reflected waveform.